Tehran University of Medical Sciences Iranian Journal of Microbiology 2008-3289 13 1 2021 02 10 8 16 Ali Pirsalehi Department of Internal Medicine, School of Medicine, Taleghani Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran Sina Salari Department of Medical Oncology, Hematology and Bone Marrow Transplantation, Taleghani Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran Ahmadreza Baghestani Physiotherapy Research Center & Department of Biostatistics, Faculty of Paramedical Sciences, Shahid Beheshti University of Medical Sciences, Tehran, Iran Ghazal Sanadgol Department of Internal Medicine, School of Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran Dorsa Shirini Department of Internal Medicine, School of Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran Maryam Moghbel Baerz Department of Internal Medicine, School of Medicine, Iran University of Medical Sciences, Tehran, Iran Saeed Abdi Basic and Molecular Epidemiology of Gastroenterological Disorder Research Center, Research Institute for Gastroenterology and Liver Disease, Shahid Beheshti University of Medical Sciences, Tehran, Iran Mohammad Esmaeil Akbari Cancer Research Center, Shahid Beheshti University of Medical Sciences, Tehran, Iran Davood Bashash Department of Hematology and Blood Banking, School of Allied Medical Sciences, Shahid Beheshti University of Medical Sciences, Tehran, Iran 2020 10 31 2021 01 23 Background and Objectives: Several studies have focused on the alterations of hematological parameters for a better understanding of the COVID-19 pathogenesis and also their potential for predicting disease prognosis and severity. Although some evidence has indicated the prognostic values of thrombocytopenia, neutrophilia, and lymphopenia, there are conflicting results concerning the leukocyte and monocyte count. Materials and Methods: In this retrospective Double­ Centre study, we reviewed the results of WBC and monocyte counts of 1320 COVID-19 patients (243 of whom (18.4%) had severe disease) both on admission and within a 7-day follow-up. Results: We found that both the number of monocytes and the percentage of monocytosis were higher in the severe group; however, it was not statistically significant. On the other hand, we found that not only the mean number of WBCs was significantly higher in the severe cases also leukocytosis was a common finding in this group; indicating that an increased number of WBC may probably predict a poor prognosis. Also, the monocyte count was not affected by age; however, univariate analysis showed that the percentage of leukocytosis was significantly greater in the older group (>50) with an odds ratio of 1.71 (P: 0.003). Conclusion: Alteration of monocytes either on admission or within hospitalization would not provide valuable data about the prediction of COVID-19 prognosis. Although the rapidly evolving nature of COVID-19 is the major limitation of the present study, further investigations in the field of laboratory biomarkers will pave the way to manage patients with severe disease better. https://ijm.tums.ac.ir/index.php/ijm/article/view/2834 https://ijm.tums.ac.ir/index.php/ijm/article/download/2834/1310